A latex article with static dissipating property

ABSTRACT

An elastomeric article, specifically an unsupported glove with two layers where the outer layer has ESD properties and its method of manufacturing is described here. The outer layer is especially formulated with a combination of fillers to reduce the surface resistance and volume resistance and helps to dissipate static charge which builds up as a result of triboelectric action. The wearer of this glove will not get any shock due to static charge which is experienced when wearing a conventional nitrile glove specially in applications such as spray painting

FIELD OF THE INVENTION

The invention relates latex articles specifically, gloves which have the ability to dissipate static charge. Industrially electrostatic dissipating gloves are used in automotive industries, in spray painting and electronic industry. Gloves that are currently in the market for the spray paint industry are mostly supported gloves most commonly with a polyurethane coating or gloves which have an electrical wire and other metal attachments that disperse electrical charge. Many of these gloves are cumbersome to the wearer due to the presence of external parts attached to it and some gloves are not quite comfortable to the wearer.

The gloves used in the electronics industry are much thinner and are normally polyurethane coated nylon or dyneema gloves.

BACKGROUND

Static electricity is generated whenever dissimilar materials move or abrade against another object. The capacity to generate static electricity of a substance depends on the condition of the surface material the objects are made out of, the dielectric constant, surface resistance and relative humidity. Further, since charge capacity is directly proportional to surface resistivity, a material with a higher surface resistivity will generate a greater static charge. Therefore in situations where static charge is generated to dissipate the charge a material with low surface resistance would be needed. The aim was to develop a product which has a lower surface resistance which will subsequently dissipate static charge. Electrostatic property can be incorporated to a glove in three ways, either by incorporating chemical additives, conductive fillers or by incorporating an inherent dissipating or conductive polymers.

The chemical additives added are usually antistatic agents; these can be either external or internal antistatic agents. External antistatic agents can be sprayed or coated into the finished article, while internal antistats are partially or completely dissolved in the latex compound. The antistat slowly “blooms” or migrate to surface and becomes electrically active. Generally antistatic agents have two halves of opposite chemical nature—a hydrophilic head and a hydrophobic tail. The hydrophobic tail avoids water but interacts with polymer. The hydrophilic head stays above surface, so antistat remains half submerged and half exposed. The hydrophilic end attracts moisture from the environment onto the plastic surface and forms a conductive layer on the surface. The conductive layer allows electric charge to flow and helps static dissipation. Although this is a cheap and an efficient way to introduce conductivity to latex articles this has a disadvantage that it could leach out during processing or remove by rubbing and washing overtime. The conductivity occurring this way varies with humidity, further due to the greasy feel on the surface antistatic agents incorporated articles may not be suitable for cleanroom applications due to the chemical contamination it could impart. Antistatic agents are used in applications such as vinyl and rubber work surfaces, polyethylene bags, polypropylene containers/trays and rubber mats.

The second method of introducing electrical conductivity to latex articles is by incorporating a conductive filler. Conductive fillers can be metal particles, metal coated glass, carbon fibers, silver powder, carbon powder or other carbonaceous materials such as graphite. When using conductive fillers the factors to be considered are particle conductivity, loading level and particle shape. At low filler loading, the composite properties remain undisturbed as the filler loading reaches a critical point, the resistivity suddenly drops. The critical point is called the percolation threshold where majority of filler particles are in contact with at least two of their nearest neighbors, thereby forming a continuous chain or network. Electrical charge passes through the composite via this network.

Usage of inherently dissipating or conducting polymers is the third method that can be used to incorporate static dissipation to an article. Inherently static dissipating polymers are generally blended with a host polymer. When blended the inherently dissipating polymers form a separate interpenetrating network within the host polymer. The advantage of using inherently dissipating polymers is its less likeness to migrate or leach out, hence can be used in clean room applications. These type of polymers are independent of atmospheric humidity unlike the antistatic agents. When using inherently conductive polymers, the polymer chain provides the conductive path for the electrons. This happens as a result of the dramatic changes on the polymer architecture such as double bonds, conjugation and doping which can change the electronic structure by oxidation and reduction. The disadvantages of using conductive polymers are the loss of stability and conductivity under ambient conditions, having poor mechanical properties and poor processability. Examples of such polymers are polyanilene, polypyrrole and polythiophene. Inherently conductive polymers are used for coating materials for films and electronic packaging.

This patent refers to use of special fillers to impart the ESD properties (electrostatic charge dissipating properties) to a glove.

PRIOR ART

U.S. Pat. No. 2,008,007 refers to a static dissipative article which had been made with a coating that comprises a surface functionalized nanoparticle component having quaternary ammine groups which acts as antistatic agents on the surface of the nanoparticles. Since the quaternary amine groups are bound to the nanoparticles migration and blooming found in typical antistatic agents is not there.

Another patent U.S. Pat. No. 5,609,969 refers to a static dissipative packaging article comprising a coating containing vanadium pentoxide, a polymer latex having a hydrophilic functionality and a nonsulfonated protective binder, this acts as an antistatic coating over an insulative polymer resin.

U.S. Pat. No. 4,828,756 refers to an electrically conductive composite comprising a conductive composite polyacetylene moiety, a nonconductive high nitrile resin and a dopant. The electrically conductive composite is prepared by insitu polymerization, it comprises the reactive products of the high nitrile resin and conductive organic polymer.

U.S. patent application 200803006200A1 refers to elastomeric articles specifically gloves made from a nitrile/natural rubber blend the surface resistance below 10⁴ Ω/sq and static decay time of less than 60 seconds. Natural rubber has been used to provide good softness and tactile sensitivity properties, however Natural Rubber does not exhibit good static resistivity. It has an outer layer that possesses antistatic properties and an inner layer that provides desired comfort and feel. Nitrile rubber is considered as a non-leachable polymeric antistatic agent, natural rubber is blended with nitrile rubber to make the polymer blend to provide antistatic property.

U.S. Pat. No. 4,769,856 refers to a antistatic fabric supported glove made with a cutout of thin antistatic polyvinyl chloride film. The cutout is adhered to the fabric using an adhesive or by stitching. The cut out covers the palm, inner thumb and finger areas of the hand This glove can be used to handle vinyl phonograph records and other objects which can be damaged by static electricity.

U.S. Pat. No. 3,594,222 relates to latex articles that uses lithium chloride as an antistatic agent in rubber latex. This is particularly used for carpets the face of which is made from fibrous textile material which tends to accumulate static charge as a result of friction on its surface. The charge can be dissipated and greatly reduced due to the antistatic agent in the latex.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide for an elastomeric article, particularly a glove, which overcomes the drawbacks of the above cited articles according to the state of the art, and particularly for allowing to provide for an article and a method for its fabrication which are fast, cheap and easy to carry out. Furthermore the elastomeric article is stable relating to its mechanical and antistatic properties and easy to dress and wear.

The present invention refers to a latex article specifically a glove made of nitrile rubber which has the ability to dissipate static charge. The nitrile rubber can be made according to a standard mix of sulphur/accelerator/zinc oxide formulation.

According to a first feature of the present invention the elastomeric article and in particularly the glove is formed by an unlined elastomeric layer of a composition comprising a nitrile rubber, particularly according to the above known standard mixture and a conductive filler.

The article and particularly the glove can be fabricated by using conventional dipping technology.

The unlined nitrile glove, according to the invention and which helps to reduce the surface conductivity of the glove and subsequently help the dissipation of static charge which could accumulate due to triboelectric action thanks to the conductive filler. Apart from being static dissipative this glove is economical, comfortable to the wearer and has good chemical resistance. This fully dipped unlined nitrile glove can be specially used in industries where paint is sprayed where static charge builds up while holding the appliance due to triboelectric action. The glove has been formulated to dissipate static charge as it forms along the glove and allowed to be earthed through human body.

DETAILED DESCRIPTION OF THE INVENTION

According to a first generic non limiting example the static dissipative nitrile article and particularly the glove is made using nitrile rubber, standard sulphur/accelerator/zinc oxide formulations with a conductive filler composed of carbon and carbonaceous materials.

The said conductive filler is provided in a quantity of 2.1% to 7.4% in weight of the standard mixture of nitrile rubber, standard sulphur/accelerator, zinc oxide, preferably in the range of 6.7% to 7.1% in weight. Typical carbon and carbonaceous materials which can be used alone or in combination are carbon composites and graphite.

A typical non limiting example of a formulation of the said nitrile rubber mixture is:

-   -   Nitrile rubber 100% in weight of the glove     -   sulphur 1.3% in weight of the glove     -   Accelerator 1.14% in weight of the glove     -   zinc oxide 4.4% in weight of the glove

The article is obtained using a coagulant dipping process which is followed by leaching and curing cycles. In this process a clean, preheated mold, more specifically a former is dipped into a coagulant that comprises of calcium nitrate and dried. Then the coagulant coated former is dipped into a latex compound that is has been compounded with all the necessary ingredients to form a glove. Then glove is cured and subsequently leached to remove the salt, dried and chlorinated.

Then the surface resistance and the static charge decay time of the glove was measured. The term “surface resistivity” is defined as the resistance measured on the surface of a material. Electrodes are placed on the surface of the material, a voltage is applied and resistance between the electrodes is measured. The ability of a material to discharge static electricity is classified according to its static resistivity. Static charge decay time measures the ability of a grounded material to dissipate a charge induced on its surface. As per the standard EN 1149 an electrostatic dissipative material should have a static decay time less than 4 seconds and a surface resistance less than or equal to 2.5×10⁹. The glove developed met the above requirements and complies with EN 1149. 

1. A dipped elastomeric article made of a latex monolayer, the said layer having static charge dissipating property.
 2. The article according to claim 1 in which the said latex monolayer is provided with a filler of conductive materials.
 3. The article according to claim 1 wherein it can be made out of elastomers made of nitrile rubber, natural rubber, neoprene, its blends or any other rubbers, SBR, Butyl, PVC, Silicon rubber, flouroelastomers.
 4. The article according to claim 1 wherein the elastomeric latex monolayer consists of nitrile latex which is a copolymer of butadiene and acrylonitrile.
 5. The article according to claim 1 in which the elastomeric latex monolayer consists of nitrile latex, standard accelerator/sulfur/zinc oxide compositions, compositions which include non-sulfur/accelerator compositions and thermoplastic elastomers along with electrically conductive material.
 6. The article according to claim 1 in which of electrically conductive material comprises at least one carbon composite.
 7. The article as claimed in claim 5 wherein the carbon composite is added as dispersion to the latex compound similar to any other solid material added to a latex composition
 8. An article according to claim 1 in which the said article is unlined.
 9. The article according to claim 1 in which the said article is a glove.
 10. The article as claimed in claim 8 wherein the said article is a disposable glove, flocklined gloves or a fabric supported glove.
 11. Use of the article according to claim 8 characterized in that the said article is used for applications where static electricity is formed through triboelectric actions on articles which handle during spraying techniques or other techniques in order to dissipate static electricity.
 12. The article according to claim 1 characterized in that the whole article, or the compounds used to make this article used to make conducting base and/or conductive pads or conductive pens for touch screen gloves.
 13. The glove according to claim 1 characterized by the following properties. Surface resistance : 7.81×104 Ω/sq Static charge decay time : <0.01 Sees Tensile Strength : 25-27 Modulus at 50% : 2.0-2.2% Modulus at 100% : 3.0-3.2% 